How to make object appear only inside a defined region
You can use Object coordinates of another mesh (set to not render) - or even an empty - to control the visibility of the text. This way when the text moves in relation to the other mesh it's visibility can be adjusted based on those translated coordinates. You can easily move and reorientate the other mesh in order to adjust the effect as desired.

Here's the material :



Note the Texture Coordinate node Object set to 'Cube'. This results in each coordinate being presented in relation to the other mesh rather than the text itself.

Adjust the Divide and Power nodes to adjust the drop-off and range of the fade-out.

This produces the following results :